efficiency calculation from non-10 fold dilutions

I have done a 5 fold dilution series with 6 points because my genes of interest are relatively low copy number and I wanted to get enough points in my dilution series to feel confident using these primers.

I am not clear how to calculate the efficiency from log base 5. My R2 values are nearly perfect (better than .998) and my slopes are around 1.4.

I thought I should use this calculation 5^(-1/m)...but the numbers I get don't seem right.